High fidelity audio system



United States Patent 3,047,661 HIGH FIDELITY AUDIO SYSTEM Daniel E. Winker, 8230 Irondale Ava, Canoga Park, Calif. Filed Jan. 18, 1957, Ser. No. 635,035 3 Claims. (Cl. 179-1) This invention has relation to an audio system in which an audio input signal is fed through an amplifier to the voice coil of a speaker. The audio output from the speaker will resemble the audio input signal with extreme fidelity due to the employment of servo-mechanism techniques. A transducer will be associated with the vibrating cone of the speaker in such a manner that the output voltage of the transducer is used as a position of reference of the actual location of the vibrating cone of the speaker. This output voltage is introduced between the audio signal input point and the amplifier but is introduced with the opposite polarity from said input signal. The input signal will be amplified by the amplifier and this amplified signal, acting on the voice coil of the speaker, will cause the speaker cone to be displaced. Displacement of the speaker cone, however, causes the transducer to be displaced and develops an output voltage at said mechanism which is fed through an appropriate feedback network to the input of the amplifier. As long as the audio input signal voltage and the positional feed back voltage from the transducer are of different magnitudes, there will be a resultant difference passing through the amplifier and this amplified difference will cause the voice coil of the speaker to drive the speaker cone in direction to produce as an output voltage of the transducer, the same voltage as is being received as an input signal. When the output voltage of the transducer and the input audio signal voltage are equal to eachother, the amplifier no longer sees any input voltage and there will be no further amplified signal to drive the output device. In other words, the speaker cone will then be in the null position.

With a high gain amplifier and a low inertia output speaker, the mechanical movements of the speaker, and consequently the audible output therefrom, will follow the electrical variations of the audio input signal with extreme fidelity, with the exception of the distortion occur-ing within the system because of the inertia of the speaker and of the system and because of any spurious oscillation developing in the amplifier.

In order to eliminate this distortion, a negative feedback which will be proportional to the rate of change of position of the speaker cone is introduced into the system before the input to the amplifier. This is, of course, the usual way of overcoming such distortion. Instead of taking this negative rate feedback from an output transformer as the normal procedure, however, it is taken instead from the speaker itself by means of an additional winding on the speaker diaphragm. There is no output transformer needed in a system of the present invention.

If a high gain D.C. amplifier with a cathode follower output is used, it is possible to couple the voice coil of the loud speaker directly to the output of the amplifier and the capacitors and output transformers which are responsible for a large portion of the distortion and attenuation of certain frequencies may be eliminated. Elimination of these components will also reduce the cost of manufacture of an amplifier made according to the present invention.

Since the cone of the loud speaker of the present invention is constantly being driven toward a null position which forces it to be directly and exactly in step with the representative of the audio input signal, it is obvious that the audible output of the system will be of extremely high quality. In a system of the present invention, this 3,647,661 Patented July 31, 1952 will be accomplished without the necessity for using extremely high quality and extremely expensive components.

In the drawings,

FIG. 1 is a partially diagrammatic and partially schematic representation of an audio aystem made according to a first form of the present invention;

FIG. 2 is a diagrammatic representation of a pair of speakers which can be substituted for the single speaker shown in FIG. 1 to constitute a second form of the invention;

FIG. 3 is a diagrammatic representation of an audio system comprising a third form of the present invention; and

FIG. 4 is a diagrammatic representation of a fourth form thereof.

In the drawings and the numerals of reference thereon, an audio input signal It is fed to an amplifier 11 where it is amplified and fed to a voice coil 12 of a loud speaker 13. The voice coil is mounted on the diaphragm 14 in the present of a permanent magnetic field 15, so that the amplified signal passing through the voice coil will cause the coil to drive the diaphragm 14 to cause the cone 16 thereof to :set up vibrations in the air which form the audible signal from the speaker.

The speaker 13 has an arm 17 of very small mass mounted on the apex of the cone 16 and adjacent the voice coil 12 so that it will move whenever the diaphragm moves. To a lower portion of this arm, there is attached the moving portion of a transducer. This transducer is denoted generally at 18. The transducer is so constructed and arranged that when there is no signal being received by the voice coil 12 of the speaker, the arm 17 will be positioned so that there is no output voltage being developed by the transducer; and when there is a signal being received by the voice coil which causes the diaphragm 14 to be deflected, there will be an output voltage developed by the transducer proportional to the amount of displacement of the diaphragm and consequently of the arm 17, from the no-signal position. This transducer output voltage is fed through an appropriate position feedback network 19 and summing network 31 to the input of the amplifier 11 in such a way that it will be degrees out of phase with or of opposite polarity to that of the incoming audio signal. This basic arrangement and conception of the invention are illustrated in FIG. 1.

According to The International Dictionary of Physics and Electronics, published by D. Van Nostrand Company in 1956, at page 919, transducers may be classified as either active or passive. An active transducer, according to this reference, is one whose output waves are dependent upon sources of power, apart from that supplied by any oneof the actuating waves, which power is controlled by one or more of these waves. It is with active transducers that we are here concerned. Examples of such active transducers are disclosed in FIGS. 1 through 4 of the drawings.

On the other hand, this source states, at page 920, that a passive transducer is one whose output waves are independent from any source of power which is controlled by the actuating waves. An example of such a transducer is a piezo-electric crystal. Such a transducer has been found to be unsatisfactory for the present purpose since the output voltage developed by it is proportional, not to the absolute displacement of the diaphragm from a null position, but rather from the rate of change of such displacement.

In FIG. 1, the means for obtaining the output voltage is disclosed as a capacitance bridge transducer. In this form of the invention, a pair of capacitor plates 32 and 33 are mechanically fixedly positioned with respect to the arm 17. A source of radio frequency signal 34 is connected to two pairs of plates 35, 35 and 36, 36 which are situated on either side of plates 32 and 33 respectively to form a capacitance bridge so that displacement of either or both of the plates 32 and 33 from position midway between their outer plates will produce a radio frequency voltage between the two plates 32 and 33. This voltage is transmitted to a demodulator 37 along wires 38 and 39. When this signal is demodulated and fed to the position feedback network 19, it will be exactly representative of the position of the capacitor plates 32 and 33 with respect to plates 35, 35 and 36, 36. This means that the signal will be exactly representative of the positioning of the diaphragm 14 or the speaker cone 16 thereof.

While the above includes the basic idea of the invention, it is recognized that certain distortion would be present in the audible output because of the inertia of the speaker and of inherent oscillations of the system. For this reason, FIG. 1 also discloses a means of deriving a degenerative rate feedback component. A feedback coil 23 is situated on the diaphragm 14 in the presence of a permanent magnetic field 15 in such a manner as to be separated magnetically and electrically from the voice coil 12. As the voice coil 12 causes the diaphragm to move, the feedback coil 28 will move at the same time in the permanent magnetic field 15 and consequently a signal proportional to the rate of change of position of the diaphragm will be fed to an appropriate rate feedback network 29. This rate feedback network signal is then fed in the proper proportion into a first summing network 30 at 180 degrees out of phase with the position feedback signal generated by the transducer 18. This combined signal is then fed in the proper proportion into the second summing network 31 as is the audio input signal from the audio signal input source 10. The signals reaching this second summing network 31 are then fed to the amplifier 11 which determines the signal fed to the voice coil 12. As the speaker cone moves, therefore, a voltage is induced in the feedback coil 28 which is proportional to the velocity of movement of said cone and coil. This voltage is summed with the input voltage from the audio signal input in the proper proportion, but 180 degrees out of phase with it in order to provide the proper damping to prevent overshoot of the cone of the diaphragm and thus to give the overall system a linear frequency response.

As is well known, certain advantages lies in the use of two speakers of different sizes to provide the audible output of an amplifier. In a second form of the invention, as illustrated in FIG. 2, a transducer 18 employing a capacitance bridge configuration is disclosed. In this form of the invention, a first loud speaker 13 and a second loud speaker 13' incorporate arms 17 and 17' respectively. A capacitor plate 32 is fixedly positioned to the arm 17 and a capacitor plate 33 is fixedly positioned to the arm 17'. A pair of outside capacitor plates 35, 35 are associated with the plate 32 while a pair of outside capacitor plates 36, 36 are associated with plate 33. A source of radio frequency signal 34 is fed to the plates 35, 35 and 36, 36 in such a way that any displacement of either the capacitor plate 32 or the capacitor plate 33 will cause a radio frequency signal to appear between the two plates. This signal is fed to a demodulator 37 along wires 38 and 39. The remainder of the circuit of the second form of the invention will be exactly as illustrated in FIG. 1. Also, the operation of the device will be the same as has been previously explained. The output from the cathode follower output circuit of the high gain direct current amplifier will be fed simultaneously to the Voice coil 12 on the loud speaker 13 and to a voice coil 12 on the loudspeaker 13'. The signal from the transducer through the demodulator 37 and the various networks illustrated in FIG. 1 will cause both diaphragms 14 and 14' to be driven to or 4 toward a null position. Consequently, the audible output will resemble the audio input signal with extreme accuracy as previously explained.

In a third form of the invention, an ionization transducer is employed. Here a pair of capacitor plates 41, 41 are mounted on the arm 17 to move with the diaphragm 14. A radio frequency signal is fed to the plates 41, 41 from a source of radio frequency signal 34. An ionization chamber 42 is mounted on the speaker base 23. As the plates 41, 41 move simultaneously to displace the chamber 42 from electrical zero, a direct current component will appear on the wires 43 and 44 which is proportional to the displacement of said chamber from said electrical zero. This signal is fed to the position feedback network 19, is combined with the signal from the rate feedback network 29, and is fed to the amplifier along with the audio input signal as previously explained.

In a fourth form of the invention, a resistance bridge transducer is employed. As illustrated in FIG. 4, a resistor 45 is mounted on the speaker base 23 in such a manner that movement of the arm 17 will cause a variation in the resistance thereof. This resistor 45 is arranged in a series loop with other resistors designated 46, 47 and 48 in that order. A regulated direct current supply 49 is connected to the loop from between resistors 48 and 45 to between resistors 47 and 46. In order to obtain a signal proportional to the displacement of the diaphragm 14 from the resistance bridge transducer, wires 50 and 51 are connected respectively from between resistors 48 and 47 and from between resistors 45 and 46 to the position feedback network 19. The signal reaching this network from the bridge network transducer will then be fed together with the degenerative feedback signal and the audio input signal to the amplifier 11.

While certain definite transducer structures have been shown and described to obtain an output voltage for practicing the present invention, it is to be understood that other active transducer structures could be employed which would be entirely suitable and within the scope of the present invention. For example, an inductance bridge transducer could be employed in which a slug would be coupled to the speaker cone through the instrumentality of an arm similar to the arm 17. An inductance coil could be provided around the slug and this coil could be fed with a radio frequency signal. Variation in the positioning of the slug would change the inductances in two halves of said coil and this change of induction could be used in producing a radio frequency voltage output which could be fed through a demodulator like the demodulator 37 to a position feedback network like that illustrated at 19 in the drawings.

What is claimed is:

1. An audio system including a source of electrical audio input signal, an audio amplifier, a loud speaker including a diaphragm member, means for maintaining a magnetic field and a voice coil fixedly mounted with respect to said diaphragm member in said magnetic field, an electromechanical transducer operably associated with said diaphragm member, said transducer having an electrical output the magnitude and polarity of which are determined by the deviation in the immediate position of said diaphragm from a neutral, non-potential position, a transducer circuit including the electrical output component of said transducer and a feedback network for inverting the phase of signals generated in said electrical output components of said transducer, an electrical connection between said source of audio input signal and the input side of said audio amplifier, an electrical connection between the output side of said amplifier and said voice coil of said speaker, an electrical connection between said feedback network and the input side of said amplifier, said transducer being constituted as a capacitance bridge transducer including first and second capacitor plates, third and fourth plates in spaced,

parallel and surrounding relation to said first plate, fifth and sixth plates in spaced, parallel and surrounding relation to said second plate, a radio frequency field set up in opposite directions between said third and fourth plates and said fifth and sixth plates, respectively, and a demodulator electrically connected between said feedback network and said transducer to receive a modulated radio frequency signal from said first and second plates.

2. An audio system including a source of electrical audio input signal; an audio amplifier; a pair of loud speakers each including a diaphra m, means for maintaining a magnetic field, and a voice coil fixedly mounted with respect to said diaphragm in said field; an electromechanical transducer having a first capacitor plate fixedly mounted with respect to a first of said diaphragms, third and fourth plates fixedly mounted in spaced, parallel and surrounding relation to said first plate, a second capacitor plate fixedly mounted with respect to a second of said diaphragms, fifth and sixth plates fixedly positioned in spaced, parallel and in surrounding relation to said second plate; a transducer circuit including a radio frequency field set up in opposite directions between said third and fourth plates and said fifth and sixth plates, respectively, a feedback network for inverting the phase of signals generated in said transducer, and a demodulator electrically connected between said feedback network and said transducer to receive a modulated radio frequency signal from said first and second plates; an electrical connection between said source of audio input signal and the input side of said amplifier; an electrical connection between the output side of said amplifier and said voice coils on said speakers; and an electrical connection between said feedback network and the input side of said amplifier.

3. The combination as specified in claim 1 and a rate. feedback coil fixedly positioned with respect to said diaphragm in said magnetic field, a rate feedback network connected to said rate feedback coil for inverting the phase of signals generated in said rate feedback coil due to change of rate of motion of said diaphragm, and an electrical connection between the output of said rate feedback network and said amplifier input.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,857 Flanders July 23, 1935 2,194,175 Wilhelm Mar. 19, 1940 2,400,953 Roys May 28, 1946 2,493,819 Harry Jan. 10, 1950 2,857,461 Brodie Oct. 21, 1958 2,860,183 Conrad Nov. 11, 1958 

